fix(assert): handle invalid regexp gracefully instead of panicking

[bk-simon]

Summary

Fixes #1794

assert.Regexp and assert.NotRegexp used regexp.MustCompile internally (via matchRegexp) when the rx argument was a string or non-*regexp.Regexp value. MustCompile panics on an invalid regexp pattern, which is documented as intended only for package-level initialization of global variables.

This causes a runtime panic rather than a test failure when a user accidentally passes an invalid regexp:

func TestExample(t *testing.T) {
    require.Regexp(t, `\C`, "some string") // panics!
}

What changed

• Changed matchRegexp signature from func matchRegexp(rx, str interface{}) bool to func matchRegexp(rx, str interface{}) (bool, error)
• Replaced regexp.MustCompile with regexp.Compile — on error the function returns the compile error
• Regexp and NotRegexp now call Fail with a descriptive "Invalid regexp: ..." message when the pattern fails to compile, so the test fails gracefully rather than panicking
• Added TestRegexpInvalidExpression to guard against regression

Test plan

• go test ./assert/ -run TestRegexp — all pass
• go test ./... — all packages pass
• TestRegexpInvalidExpression specifically verifies invalid patterns fail gracefully (not panic)

[Copilot]

Pull request overview

This PR fixes a panic issue in assert.Regexp and assert.NotRegexp when invalid regexp patterns are provided. Previously, these functions used regexp.MustCompile, which panics on invalid patterns. Now they gracefully fail the test with a descriptive error message.

Changes:

• Modified matchRegexp to return an error instead of panicking on invalid regexp compilation
• Updated Regexp and NotRegexp to handle compilation errors gracefully
• Added regression test to verify invalid patterns fail tests without panicking

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
assert/assertions.go	Changed matchRegexp to return error, replaced regexp.MustCompile with regexp.Compile, added error handling in Regexp and NotRegexp
assert/assertions_test.go	Added TestRegexpInvalidExpression to verify invalid patterns fail gracefully

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